There are several known processes for producing a porous PTFE material by expanding a paste extruded unsintered PTFE article including (1) a process comprising paste extruding PTFE fine powder having a crystallinity of at least 95% at a temperature lower than the melting point of the powder, extracting an extrusion lubricant, drying the article and expanding the article at a temperature lower than the crystalline melting point of PTFE (cf. Japanese Patent Publication Nos. 44664/1973 and 18991/1976), (2) a process comprising heating a sintered PTFE at a temperature of more than 327.degree. C. and then cooling it very slowly to improve the crystallinity of PTFE, and expanding the PTFE article (Japanese Patent Publication No. 42794/1978) and (3) a process comprising sintering a PTFE film at a temperature not lower than 327.degree. C. and expanding the sintered sheet across the calendering direction (Japanese Patent Publication (unexamined) No. 55378/1978).
According to the process (1), the resultant PTFE material may be biaxially stretchable and has a large tensile strength and a very low apparent specific gravity. However, in this process, the PTFE fine powder used should have a crystallinity of at least 95%. The expanded porous PTFE material should be heat set at a temperature higher than the melting point of PTFE and the surface of the unsintered porous PTFE material is tacky and easily contaminated.
In the process (2), the heat set step is not necessary and the produced porous PTFE material has superior gas permeability to an unstretched material. However, the sintered PTFE material can be stretched only monoaxially and the draw ratio is as low as about four. The process comprises many steps including at least two heating steps so that it requires a large amount of heat energy and a long time. The produced porous PTFE material has a comparatively high apparent density.
In the process (3), the sintered PTFE sheet is stretchable only in a direction across the calendering direction at a temperature lower than 250.degree. C. at a draw ratio smaller than 4.2. The produced porous PTFE material has a comparatively high apparent density.